The invention relates to electrical dc-machines of the kind having a stationary magnetic circuit with an air gap and a cylindrical, ironless, self-supporting rotor coil mounted for rotation in the said air gap. More particularly, the invention concerns a rotor coil for use in such a machine and methods of making the same.
Rotor coils with which the present invention is concerned comprise a plurality of coil sections connected in series with each other, the ends of each coil section being adapted for connection at the respective commutator segments of the dc-machine. The coil is generally formed by at least one layer of wire, the wire comprising a conductive core, an insulating layer surrounding that core and an outer layer comprising a supporting material capable of being softened and of subsequently solidifying in order to provide a mechanical support for the wound coil.
In dc-machines of the kind referred to above and more particularly in small dc-machines, the lifetime of the machine is determined by the lifetime of the commutator. A commutator comprises a number of segments connected to the coil and brushes co-operating with the said segments. When a brush is passing from one commutator segment to the following one, the self-induction of the coil generates voltage peaks and these voltage peaks lead to generation of sparks between the brush and the commutator segments. These sparks are at length damaging the commutator and considerably reduce the lifetime thereof. This is particularly undesired in the case of micromotors where the lifetime of the whole motor depends on the lifetime of the commutator.